AVI is pretty much raw footage and will take a lot of space, which also means you need a hard drive of decent speed. MP4 is more compressed but will take more out of your cpu, unless you are using cuda or amd app with Nvidia or AMD graphics cards. If you want to use MP4 with your CPU then choose the quicksync option. That will give you much better performance. If there is no quicksync option, your CPU may not be capable of it, or it is not enabled. Quicksync is only available in newer Intel processors. What are your specs?